@jcarrolo01
Osclass 4.1.1 use may use different version of PHP mailer (latest of version 5.x.x), would like to update to 6.x.x in osclass 4.2, but they have changed file structure in this library and based on experience in 3.9 update, it may affect plugins those include files from library direclty.

I am having major problem also.  The problem did not exist with 3.8.

Now all of my emails are phpmail only.  Other send email options do not work.

Problem is gmail does not like the header from phpmail and puts all of my emails into customers spam folder.  Big problem  All email validation goes to spam.

Below is the header problem.

Return-Path: <[email protected]>
Delivered-To: [email protected]
Received: by server.rosinto.com (Postfix, from userid 1000)
   id 3E87D3E842; Wed, 21 Oct 2020 14:53:06 +0000 (UTC)
To: SMA San Miguel Allende Classified Ads FREE <[email protected]>
Subject: [SMA San Miguel Allende Classified Ads FREE] Contact -
X-PHP-Originating-Script: 1000:class.phpmailer.php
Date: Wed, 21 Oct 2020 09:53:06 -0500
From: Steve Imhof <[email protected]>
Reply-To: [email protected]
Message-ID: <[email protected]>
X-Mailer: PHPMailer 5.2.27 (https://github.com/PHPMailer/PHPMailer)
MIME-Version: 1.0
Content-Type: text/html; charset=utf-8

What is problem in header?
Header is constructed by PHP mailer itself

Important notice is here (I did not get such issue when using gmail, maybe smtp auth is needed or so?)
https://stackoverflow.com/questions/9988325/everytime-my-mail-goes-to-spam-in-phpmailer

Based on you code i notice that you are sending an email directly from you web page on your domain.

For example you used an @hotmail.com address.

When the recipient receive the emails the mail service of the recipient may test a reverse DNS of the sender of the mail. So the sender is from @hotmail.com but the mail comes from your domain which of course is not hotmail.com.

So I receive a mail from an address @hotmail.com but the IP sender isn't related at all with domain hotmail.com: that's SPAM!

I have the same problem. I have two sites on the same hosting provider.
One site sends all the mail "register, publish, update, etc.", and the second site sends only "register, publish" and that's it.
Both sites are osclass 3.9, Osclass configured the same. Which does not send all mail has been updated to 4.1.1. Nothing changed.
I checked everything, I still don't understand why

It would be good to find what is in logs, if it is not sent at all or it is filtered by inbox mail...

Generally not sent from the very construction of the site. One domain name was changed to another. Later I made a second site, transferred a backup of the first one to it. Database and directory. And all mail works on it. On the first site, the mail has not been working completely until now

@andrey
So does it work now? if so, it would be good to write guide for other people having same issue

Now I accidentally noticed that when registering, the user is not sent a confirmation email.
I tried to register from different mail domains (gmail.com, yandex.ru), none of them received a confirmation letter. Then I checked out new users, there were many who could not confirm. Approximately 50/50 the user doesn't work.
Activated user manually and admin panel. Placed an ad from this user, the letter also did not come to the user that "the ad was successfully published"
In this case, letters about publication and registration come to the administrator
Before the update, the occlass did not encounter this problem. What to do? Thanks.

Enable debug log and get details what happened

Nothing, empty in debug.log
Neither log nor PHP errors

I will test "the ad was successfully published" mail tomorrow, I have mostly no plugins on test site.

Some nonsense. The smtp checkbox was removed and sending mail began to work as expected. On version 3.9 it did not work like that, so I did not even try to turn off the smtp

Although my second site with version 4.1 and configured mail as it was on 3.9 works further without problems. Email on one smtp domain as the first. Miracles

Almost 800 letters arrived that were not sent to the admin for the whole time. Somewhere in the class were stored as a draft, but there were no php errors

@Andrey
I've done small research and everything works as expected.
- "Edit options for the listing {ITEM_TITLE}" email comes if user is not logged in
- "A new listing has been published" comes only to admin, if this is enabled in listing settings
- "Validate your listing" comes is listing validation is enabled